The present invention relates to gas turbine engines and more particularly, to methods and apparatus to facilitate controlling gas turbine engine bypass airflows.
At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, an aft fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, and a high pressure turbine. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases which are discharged downstream to a low pressure turbine that extracts energy there from for powering the forward and aft fan assemblies.
In addition to flow considerations, gas turbine engines are also designed to meet stringent noise, weight, and performance requirements. One known type of gas turbine engine is a variable cycle engine (VCE) that is operable in a double bypass mode. More specifically, flow modulation potential is increased by splitting the fan into two sections, wherein each section is in flow communication with a separate concentric bypass duct surrounding the core engine. During operation, the bypass ratio, i.e., the ratio of the quantity of airflow bypassing the core engine to the quantity of airflow passing through the core engine can be varied by selectively mixing or separating the bypass duct stream and the core engine exhaust stream through various systems valuing and mixing systems.
To facilitate enhanced control of the bypass ratio, at least one known VCE includes a nozzle assembly that includes an internal nozzle valve or plug for controlling airflow through the bypass ducts. More specifically, in such engines, the internal plug is supported by a plurality of circumferentially spaced, radically extending support struts that are coupled to the outer duct structure of the engine. As such, because these support struts are positioned within the flow path of the exhausted core engine airflow, the support struts are exposed to high temperature airflows and may be subjected to large thermal gradients generated from the hot core engine airflow and the cool bypass airflow. Over time, the support struts may be prematurely fatigued of damaged due to continued exposure to high operating temperatures generated by the gas turbine engine.